SM8224C

SEIKO NPC CORPORATION —9

FUNCTIONAL DESCRIPTION

The SM8224C receiver with caller ID service conforms to the Bellcore standards. It incorporates FSK demod-

ulator and dual-tone detection functions in a single chip. It has a dedicated dual-tone signal input so that the

FSK signal input amplification and dual-tone signal input amplification can be set independently. This allows

systems can be easily constructed that provide pre- and mid-conversation information services.

FSK Demodulator

Calling number identification service is sent as an FSK signal, and the SM8224C FSK demodulator processes

this signal. The FSK signal conforms to the following Bellcore standard.

Note: S1, S0, MODE setting should be used from above combination except Test mode.

Dual Tone Detector

When using mid-conversation information services, 2 mixed signals of 2130Hz and 2750Hz are sent on lines

L1 and L2. The SM8224C detects the 2 signals from the background noise. It uses 2 separate high-order filters

with center frequencies of 2130Hz and 2750Hz to detect the presence of the signal frequencies.

In series with the filters are level detectors. When the input level exceeds the preset rating, the signal is

detected. When the input level is below the rating, the signal is not detected.

If both the 2130Hz and 2750Hz signals are simultaneously detected, DTDET goes HIGH and starts charging

the time constant circuit comprised by an external capacitor and external resistor. When the time constant cir-

cuit voltage rises above a fixed voltage level, STR signal goes LOW to indicate dual tone detection.

Table 1. FSK signal

Parameter

Description

Modulation type

Continuous-phase binary frequency-shift-keying

Logic “1” data (mark)

1200 ± 12 Hz

Logic “0” data (space)

2200 ± 22 Hz

Signal level (mark)

−32 to −12 dBm

Signal level (space)

−36 to −12 dBm

Data transfer rate

1200 ± 12 baud

Table 2. Function select

S1

S0

MODE

Function

STR

LOW

LOW

HIGH

Power-down

HIGH

HIGH

LOW

LOW

1. DT1, DT2, DTQ are active in this mode only.

LOW (dual tone confirmation)

LOW

HIGH

LOW

FSK and dual-tone detection from

TIP/RING

LOW (dual tone confirmation)

HIGH

HIGH

LOW

FSK detection from TIP/RING

HIGH

LOW

LOW

LOW

2. Test mode should not be used.